The invention is directed to electric power steering systems, particularly towards a torque ripple free system for electric power steering.
Electric power steering (EPS) has been the subject of development by auto manufacturers and suppliers for over a decade because of its fuel economy and ease-of-control advantages compared with traditional hydraulic power steering. However, commercialization of EPS systems has been slow and is presently limited to small and midget-class cars due to cost and performance challenges. Among the most challenging technical issues is the annoying pulsating feel at the steering wheel and the audible noise associated with the type of high performance electric drives needed to meet the steering requirements.
The so-called xe2x80x9cTorque-Ripple Freexe2x80x9d (TRF) system is an advanced electric drive concept, which was devised with the specific objectives of reducing the motor torque pulsation and audible noise in electric power steering systems to ultra-low levels comparable to those experienced in cars equipped with hydraulic power steering systems. In theory, and under ideal conditions, the new concept should produce no torque ripple at all, and hence the name. However, due to practical limitations discussed below, some low levels of torque ripple unavoidably remain.
The key elements of the TRF electric drive concept include:
(a) A sinusoidal drive to reduce commutation torque ripple to ultra-low levels. This is accomplished by:
(1) A sinusoidal inverter/controller to switch currents into the three-phases to be in synchronism with the rotor at all times. The currents are controlled to have sinusoidal waveforms
(2); Permanent magnets designed to produce a sinusoidal magnetic field distribution in the windings region of the machine, thus inducing voltages in the machine windings of sinusoidal waveforms, and
A low cost, higher resolution position sensor to provide the controller with accurate rotor position information, necessary to control the current waveform to the desired sinusoidal shape.
(b) An electrical machine with slotless winding to eliminate both cogging torque and pulsating radial forces. FIG. 3 illustrates a schematic of the TRF motor cross-section for illustration.
(c) A composite iron stator yoke to replace laminated yoke and eliminate its associated whining noise. It simultaneously functions also as a housing for the stator, thus reducing the stator manufacturing cost and defraying some of the added cost of the more expensive magnet and position sensor required for this concept.
(d) A composite reinforced plastic rotor core and shaft, which is another cost saver because it can be molded to shape instead of machined. In addition it will help with lowering the rotor inertia due to its lower density compared with steel, thus compensating for the increased inertia due to a needed larger magnet. While using a plastic rotor core tends to reduce its torsional stiffness, it is found to yield acceptable angular strain.
(e) A gear reduction box with a high gear ratio to reduce the motor size and cost.